vgabios: Simplify planar4 vgafb code.

vgabios: Simplify planar4 vgafb code.

Don't bother programming the vga registers to manipulate the
framebuffer when in planar4 mode.  Instead, just switch between the
four planes and do the manipulation with regular reads and writes.
This makes the code simpler to understand (the vga hardware
manipulations are arcane and complex).

Note, this could make text scrolling in planar4 mode slower and more
likely to result in tearing.  However, it's unlikely anything
important uses the vgabios in planar4 mode.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>